In relation to a wireless communication system, such as a cellular system, improvement of frequency use efficiency for an OFDM (Orthogonal Frequency Division Multiplexing) scheme has been studied. Specifically, the improvement is achieved by applying adaptive modulation or frequency scheduling to each of resource blocks into which a plurality of sub-carriers are bundled. Adaptive modulation is a scheme for determining a coding rate and a modulation scheme so as to satisfy a predetermined packet error ratio depending on a state of a propagation path observed by a receiving side. The scheme is also called adaptive MCS (Modulation and Coding Scheme) control. Further, frequency scheduling is achieved through the following operation. Namely, a plurality of mobile stations of a wireless communication system report states of propagation paths in respective resource blocks observed by receiving sides. A base station collects the states of the propagation paths and allocates favorable resource blocks to the respective mobile stations according to a predetermined scheduling algorithm. A value of a report on a state of a propagation path used for adaptive modulation or frequency scheduling is called channel information, i.e., Channel State Information (CSI).
In order for the CSI report, a reference signal used for acquiring various indices of a propagation path or a transmission signal is introduced into the wireless communication system, such as a cellular system. FIG. 11 is an explanatory operation diagram schematically showing CSI reporting operation performed when adaptive modulation and frequency scheduling are carried out. A reference signal is transmitted from a base station (nodeB) that is a transmission device to terminals (UE A, UE B) that are reception devices, and a CSI report is transmitted from each of the terminals to the base station. The base station performs adaptive modulation and frequency scheduling based on a reported CSI from each of the terminals, and then allocates terminals to respective resource blocks and determines a coding rate and a modulation scheme.
For instance, a reference signal (Reference Signal: RS) is employed for LTE (Long Term Evolution) of a next generation communication system discussed by the 3GPP (3rd Generation Partnership Project) that is an international standard setting organization of mobile communication. During downlink communication from the base station to the terminal, a reference signal to be transmitted from a transmission device (the base station) to a reception device (the terminal) is used primarily in use applications; for instance, (1) estimating a propagation path for demodulation purpose and (2) measuring quality of adaptive modulation or frequency scheduling. In LTE, a multi antenna system to which MIMO (Multiple Input Multiple Output) is to be applied transmits a reference signal in units of predetermined wireless resources. In LTE-advanced (hereinafter called “LTE-A”) that is a communication system resultant of further advancement of LTE, introduction of high-order MIMO (employing; e.g., eight transmission antennas), coordinated multipoint (CoMP) transmission/reception, and the like, have been discussed to achieve further advancement. In order to address upgrading of frequency use efficiency, such as that mentioned above, there is required measuring a state of a propagation path with a higher degree of accuracy.
In addition to the channel information (CSI), inter-cell interference is mentioned as another factor that contributes to control of adaptive modulation or frequency scheduling. Specifically, whether or not the reception device can detect and demodulate a signal without failure by means of a resource block allocated a transmission device, a coding rate, and a modulation scheme depends on intensity of a desired signal against noise and interference; namely, an SINR (Signal-to-Interference plus Noise Ratio). Therefore, it is desirable that the reception device should measure inter-cell interference affected by other cells and should report intensity of a desired signal against the inter-cell interference as channel information. Incidentally, other cell signals that actually interfere with the desired signal fluctuate according to states of allocation of data to other cells.
FIG. 12 is an explanatory operation diagram schematically showing inter-cell interference and a status of traffic with other cells. When a signal is transmitted from a base station (nodeB) that is a transmission device to a terminal (UE A) that is a reception device, the terminal (UE A) receives, as an interference signal, a signal originated from a base station (Interfering nodeB) for another cell in place of a desired signal to be acquired. When the terminal observes an interference signal (another cell signal) from another cell in order to measure inter-cell interference, there mixedly exist a case where data are allocated (white blocks in the drawing) and another case where data are not allocated (hatched blocks in the drawing) depending on observation timing and a frequency resource to be observed. In ordinary operation, a terminal does not have any means for grasping control information about allocation of data to another cell; hence, the terminal cannot ascertain a state of allocation of another cell signal. An averaging procedure over a long time and a wide range is mentioned as means that enable easing of influence caused by a change in state of data allocation.